1. Field of the Invention
The present invention relates to a battery pack containing a secondary battery integrally with a processing circuit to perform processes including protection against anomaly occurrence in the secondary battery, a battery protection processing apparatus to perform its processes, and a control method of the battery protection processing apparatus.
This application claims priority of Japanese Patent Application No. 2003-385372, filed on Nov. 14, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
In recent years, there are an increasing number of portable electronic devices such as digital video cameras on the market. A great importance is attached to the performance of secondary batteries mounted on these electronic devices. Such secondary batteries include lithium-ion batteries.
In particular, if a lithium-ion secondary battery is overcharged, lithium ion separates out as lithium metal at a negative electrode. It is known, in the worst case, that the battery smokes, ignites, or explodes. If the battery is over-discharged, the electrode inside is subject to a small amount of short-circuiting or capacity degradation. When the positive and negative electrodes are short-circuited, it is also known that an overcurrent flows to cause abnormal heating. In order to prevent overcharging, over-discharging, short-circuiting (overcurrent), the lithium-ion secondary battery is generally provided with a protection function to monitor these abnormal states and a switch to prevent the abnormal states.
FIGS. 1A and 1B present graphs showing voltage and current changes when discharge and overcurrent occur in a lithium-on secondary battery.
FIGS. 1A and 1B show an example of lithium-ion battery cell used for digital video cameras and digital still cameras for home use. A fully charged voltage is assumed to be 4.2 V, and an over-discharge detection voltage is assumed to be 3.0 V. FIG. 1A shows changes of the battery cell voltage during discharge of 2 W power consumption. As shown in FIG. 1A, the battery cell voltage decreases down to the over-discharge detection voltage after approximately 90 minutes from the fully charged state. If a discharge load is released, the battery cell voltage temporarily increases, but gradually decreases thereafter due to self-discharge. If the battery is left unused for a long time, the battery cell voltage decreases to 0 V. When the positive and negative electrodes are short-circuited, the battery cell voltage momentarily decreases to approximately 1 V as shown in FIG. 1B. At this time, an overcurrent of approximately 15 A flows.
On the other hand, a remaining battery capacity display function is increasingly provided for the above-mentioned portable electronic devices using the secondary battery as a power supply. As shown in FIG. 1A, especially in the lithium-ion secondary battery the battery cell voltage gradually and linearly decreases except immediately before and after the discharge. Accordingly, the use of only the battery cell voltage cannot accurately detect the remaining battery capacity. It becomes possible to accurately calculate the useful remaining life by using accumulated values of a charge and discharge current, the battery cell temperature, and the like. In order to realize such remaining battery capacity display function, there is commercially available a battery pack that contains the secondary battery and circuits such as a microcontroller in the same package.
FIG. 2 shows an internal configuration example of a conventional battery pack.
The conventional battery pack in FIG. 2 comprises: a battery cell 1 comprising a lithium-ion secondary battery; protection switches SW11 and SW12 for discharging and charging control, each comprising a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) equivalently containing the diode between the source and the drain according to the structure; resistor Rs for current detection; a battery protection circuit 110; a microcontroller 120; a clock oscillator 130 for microcontroller operations; a thermistor 140 to detect temperature of the battery cell 1; and a communication I/F (interface) 150 to establish communication with an electronic device provided with this battery pack.
In the battery pack, the protection switches SW11 and SW12 each comprise an FET and a diode. The protection switch SW11 can turn off discharge current. The protection switch SW12 can turn off charge current. Accordingly, when the battery cell 1 is charged, a charger is connected to a positive electrode terminal Eb1 and a negative electrode terminal Eb2. In addition, the protection switch SW12 is turned on. The positive electrode terminal Eb1 and the negative electrode terminal Eb2 may connect to a device functioning as discharge load. In this case, turning on the protection switch SW11 can supply power to the device. The battery protection circuit 110 is also integrated with various circuits for supplying power to the microcontroller.
The microcontroller 120 is a circuit to calculate information needed to display the remaining capacity of the battery cell 1 and operates on the power supplied from the battery protection circuit 110. For stable operations, the battery protection circuit 110 controls the startup timing. The microcontroller 120 computes necessary information under software control based on digitized values equivalent to charge and discharge currents and battery cell voltages supplied from the battery protection circuit 110 and on temperature values detected by the thermistor 140. The microcontroller transmits the information to the electronic device mounted with the battery pack via the communication I/F 150 and a control terminal 4. This makes it possible to display the remaining capacity of the battery in the electronic device body.
As mentioned above, however, the battery cell voltage of the secondary battery greatly varies with conditions. On the other hand, the microcontroller system is designed on the premise that the power supply voltage is stably supplied to the microcontroller. For this purpose, as shown in FIG. 2, the conventional battery pack uses another circuit independent of the microcontroller to provide the protection function that monitors anomalies such as overcharge, over-discharge, and overcurrent of the secondary battery. There is an example of such circuit that mainly comprises a special voltage comparator as a major component to implement the battery cell protection function (e.g., see Japanese Patent No. 3136677 (paragraphs [0011] through [0016], FIG. 1)).
FIG. 3 schematically diagrams battery cell states of the conventional battery pack.
As shown in FIG. 3, the conventional battery pack maintains a normal state when the voltage of the battery cell 1 ranges from 3.0 to 4.25 V, for example. In this state, both the protection switches SW11 and SW12 turn on to enable both the power supply against discharge loads and charge operations for the charger, if connected. When the voltage of the battery cell 1 exceeds 4.25 V, an overcharge state occurs. The protection switch SW12 turns off to disable the charge. When the voltage of the battery cell 1 is lower than 3.0 V and is higher than or equal to 2.50 V, an over-discharge state occurs. The protection switch SW11 turns off to disable the discharge. In this state, however, the power supply to the microcontroller 120 continues, keeping the microcontroller 120 operating.
When the voltage of the battery cell 1 becomes lower than 2.50 V, all the discharge stops to prevent the capacity degradation of the battery cell 1. Consequently, the microcontroller 120 stops operating. Thereafter, applying a voltage from the charger terminal starts charging the battery cell 1. When the voltage exceeds a specified value, the microcontroller 120 starts operating.
The current detection resistor Rs is used to detect a discharge current. When the discharge current exceeds 3.0 A, an overcurrent state occurs. The protection switch SW11 turns off to inhibit the discharge. This state also stops operations of the microcontroller 120 and the like. Releasing the discharge load automatically resumes the normal state.
As mentioned above, the conventional battery pack is independently mounted with the protection circuit for the lithium-ion secondary battery and the microcontroller to compute display of the remaining battery capacity. Recently, by contrast, it is expected to mainly use the microcontroller to implement the above-mentioned function of the protection circuit and integrate most of the circuits on a single semiconductor circuit board from the viewpoint of miniaturization, decreasing the number of parts, and reducing parts costs.
As mentioned above, however, secondary battery voltages are unstable depending on situations. The microcontroller itself is not stably supplied with the power supply voltage. It has been difficult to monitor secondary battery anomalies mainly under software control of the microcontroller. If the microcontroller realizes part of the protection function, it is mainly implemented by special hardware such as a voltage comparator. The microcontroller is used as a supplementary function for that hardware.
When the microcontroller mainly implements the protection function for the secondary battery, it is important to save as much power consumption of the microcontroller itself as possible and stably supply the power to the microcontroller.
The battery pack may use a plurality of serially connected battery cells depending on the magnitude of loads applied to a connected device. In this case, it is necessary to individually determine overcharge and over-discharge states for each of the battery cells. When only the voltage comparator is used to detect battery cell voltages as mentioned above, however, it is necessary to provide a protection circuit containing as many voltage comparators as the serially connected battery cells, causing problems of increasing design costs and enlarging the installation space.